This invention relates to a video display and in particular to a cathode ray tube display operable in accordance multiple high definition standards.
The Society of Motion Picture and Television Engineers (SMPTE), Advanced Television Standards Committee (ATSC) and the Federal Communications Committee (FCC) formulated and adopted standards for the United States which describe both standard definition (SD) and high definition (HD television services. The resulting ATSC document (Doc. A/53B) defines various video signal parameters and a wide variety of scanning standards. For example in a particular high definition standard, published by the SMPTE as 274M-1998, parameters are defined for eleven varieties of scanning standards each having the same constant image format (CIF) of 1920 active samples in a horizontal direction and 1080 active lines vertically per frame. Exemplary parameters abstracted from SMPTE 274M-1998 are shown in Table 1 of FIG. 1. In addition each frame has the same total number of lines regardless of the frame rate. Although standard SMPTE 274M-1998 defines identical image formats with nominally identical clock frequencies, eleven standard varieties result as a consequence of differing frame rates and progressive or interlaced image construction.
In SMPTE standard 296M-2001 parameters are published which define a family of eight progressively scanned standards having a constant image format of 1280 active pixels horizontally by 720 active lines vertically. In the description that follows, analog video display consequences are explained, and inventive solutions discussed with reference to the exemplary parameters of Table 1. However, substantially similar analog display consequences result with signals defined by SMPTE 296M-2001 and advantageously, the inventive solutions which follow are equally applicable to standards defined by SMPTE 296M-2001.
The standardized requirement of SMPTE 274M-1998 for constant image formats with identical clock frequencies results in a nominally constant time period for image readout and display. Furthermore with each frame constructed from identical numbers of lines, the differences in frame frequencies between, for example 25 Hz and 30 Hz, must be accommodated by differences in blanking periods. For example, if a 60 Hz interlaced standard is considered as shown at line 1 of Table 1, FIG. 1, the duration of an active frame is 1920xc3x971/74.25 MHzxc3x971080, or approximately 27.9272 milliseconds. The total duration of one frame of a 60 Hz interlaced standard is 33.3333 milliseconds thus a period of 33.3333xe2x88x9227.9272, or approximately 5.4061 milliseconds is available for synchronization and blanking. If a 50 Hz interlaced standard is considered, line 2 of Table 1, FIG. 1, the duration of an active frame is 1920xc3x971/74.25 MHzxc3x971080, or approximately 27.9272 milliseconds however total duration of one frame of a 50 Hz interlaced standard is 40.0000 milliseconds, thus a period of 40.0000xe2x88x9227.9272, or 15.0728 milliseconds is available for synchronization and blanking. Since the numbers of active lines and total lines per frame, 1080, 1125 respectively, are the same for each of the eleven standards defined by SMPTE 274M, the difference in horizontal and frame duration between 50 Hz and 60 Hz signals can only be accommodated by modification or variation of horizontal blanking periods.
This blanking or non-active picture time accommodation is apparent when the total number of samples per line is considered. For example, an interlaced 60 Hz, 30 frame per second standard has 1920 active pixels or samples and a total pixel count of 2200, which provides 280 clock periods for horizontal blanking and synchronization. However, an interlaced 50 Hz, 25 frame per second standard has 1920 active pixels and a total horizontal pixel count of 2460, thus 720 clock periods are available for horizontal blanking and synchronization. For example if similar intervals are required by the display circuitry to perform blanking and synchronization functions, then an additional 440 clock periods are available per line. Thus with 440 extra samples or clock periods per line, the 25 Hz standard can be constructed with identical active picture areas to that of the 60 Hz standard but with different horizontal and frame rates. For example, with a total number of 1125 lines per frame and 440 additional clock samples (1/74.25 MHz) per line, an additional period of approximately 6.6666 milliseconds elapses, which when added to the nominal period of a 30 Hz frame rate signal to yields an identical image format but with a 25 Hz frame rate.
In an exemplary video display, or television camera view finder, operable at various of the standards defined by SMPTE 274M it is conventional to control the deflection amplitude and maintain the same retrace or fly-back time during operation in each standard. However, such display design constraints result in active picture areas of differing sizes when selecting between 30 Hz and 25 Hz frame rate standards. If an exemplary 30 Hz image is arranged to nominally fill the viewable tube face then active picture area of a 25 Hz image will be presented with reduced width. In fact all 1920 active pixels of the 25 Hz frame rate image will be displayed adjacent to 440 black pixels or blanking samples. The visual effect is that a 25 Hz active area is displayed with about 82% of the width of a 30 Hz image, even though SMPTE 274M standard provides for a constant image format.
Clearly when operating at a 25 Hz frame rate there are various remedies that can be applied to restore the compressed display image width. For example the active portion of each line can be electronically stretched to eliminate the 440 blanking pixels. Such processing though not technologically difficult represents a standards specific cost increment for the display, and additional power dissipation which is a specific concern when battery powered operation is considered. Similarly the horizontal deflection amplitude of the display can be increased such that the scanning electron beam is deflected to each tube edge in the period of the active video, rather than in the duration of the total line pixel count less retrace time. Such an increase in deflection amplitude results in additional power dissipation which also shortens battery operation time.
It is clearly desirable that an exemplary video display, operable at various of the HD standards, is capable of providing active picture displays of substantially similar sizes without aspect ration distortion, and without significantly increased material cost or power dissipation.
In an inventive arrangement a cathode ray tube display is operable to display images having identical active image pixel counts and a plurality of different scanning frequencies. The inventive arrangement comprises a deflection arrangement forming a raster to display the active image pixels on the cathode ray tube. A power supply for the deflection arrangement is controlled to vary in a manner which facilitates the display of substantially all the active image pixels with a substantially similar picture width when operating at different ones of the plurality of scanning frequencies.
In a further advantageous arrangement a video display with a cathode ray tube is operable at a plurality of scanning frequencies. The video display comprises a deflection arrangement forming a raster of substantially constant width on said cathode ray tube when operating at ones of the plurality of scanning frequencies. A power supply is coupled to the deflection arrangement and is controlled to maintain a substantially similar deflection current generated by the deflection arrangement when operating at each one of said plurality of scanning frequencies.
An inventive method for the display of raster scanned images having a plurality of scanning frequencies, the method comprises selecting a retrace time for raster scanning in accordance with each one of the plurality of scanning frequencies; and, maintaining a substantially constant active picture width on the display for images having ones of the plurality of scanning frequencies.
In a further inventive method for raster scanned display of images having a plurality of scanning frequencies. The method comprises selecting a retrace time for the raster scanning in accordance with each one of the plurality of scanning frequencies; and, controlling deflection current for the raster scanned display to be substantially equal in each one of the plurality of scanning frequencies.